flatten a nested list (turn lists within lists into a single one).

// Challenge: flatten a nested list (turn lists within lists into a single one).
// Input: [1, [2, 3], [4, [5, 6]]]
// Expected output: [1, 2, 3, 4, 5, 6]

// Solution: recursion.
// We walk through each element; if it's a list, we flatten it too
// (calling ourselves) and add its elements.
List<int> flatten(List<dynamic> list) {
  final result = <int>[];
  for (final element in list) {
    if (element is List) {
      // It's a sublist: we flatten it recursively.
      result.addAll(flatten(element));
    } else if (element is int) {
      result.add(element);
    }
  }
  return result;
}

void main() {
  print(flatten([1, [2, 3], [4, [5, 6]]])); // [1, 2, 3, 4, 5, 6]
  print(flatten([[1], [2], [3]])); // [1, 2, 3]
  print(flatten([1, 2, 3])); // [1, 2, 3] (already flat)
  print(flatten([])); // [] (edge case)

  // Explanation:
  // Recursion is the key: when we find a list inside the list, we call
  // flatten on it again. That works no matter how many levels of nesting
  // there are. The "is" operator checks the type at runtime to decide
  // whether we flatten or add.
}